1. Technical Field
The present invention relates to a piezoelectric speaker for use in an acoustic device and the like, and a method for manufacturing the same.
2. Background Art
Conventionally, a piezoelectric speaker, known as a compact and low-current driven acoustic device in which a piezoelectric material is used as an electro-acoustic transducer element, is used as an acoustic output device for use in a compact device (for example, see Patent Document 1). In general, the piezoelectric speaker has a structure in which a piezoelectric element having formed thereon an electrode of silver thin film or the like is adhered to a metal diaphragm. A sound output mechanism of the piezoelectric speaker is such that an application of an AC voltage to both surfaces of the piezoelectric element causes a generation of a shape distortion of the piezoelectric element, so that the metal diaphragm is vibrated, thereby generating a sound.
On the other hand, a low-frequency reproduction capability of a speaker unit greatly depends on a volume of air to be released using an amplitude of vibration of the diaphragm included in the speaker unit. Therefore, in order to increase a reproduction bandwidth at a low-frequency side in the speaker unit, the speaker unit has to be constructed so as to increase an area of the diaphragm or increase a stroke of the diaphragm. However, when the speaker unit is mounted on a compact device or the like, the speaker unit is required to have a restricted diameter and thickness depending on a volumetric capacity of a casing of the device and other devices to be mounted thereon.
In recent years, while a market trend requires reductions in thickness, size, and weight of a mobile device such as a mobile telephone, an acoustic system mounted thereon is required to perform both high quality sound reproduction and stereo reproduction, resulting in the required number of speaker units being plural for realizing such reproductions. However, it is difficult to increase the sizes of the casing and the speaker against the trend or to leave the size of the device having a plurality of speaker units mounted thereon unchanged from a current size. Under such circumstances, although the high quality sound reproduction is expected, the compact device has mounted thereon an electrodynamic small-diameter micro speaker which provides a poor low-frequency reproduction performance.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-16692
Here, as one of indexes used for determining a capability represented as an acoustic characteristic of a speaker unit, a lowest resonant frequency (hereinafter, referred to as an F0) is used. The lowest resonant frequency F0 is obtained using equation (1) as follows.P=(2πf)2×ρ0a2/2πl  (1)where P: sound pressure (N/m2), a2: effective vibration area (m2), ρ0: air density, f: frequency (Hz), x: amplitude (m), l: measured distance (m).
According to equation (1), when a speaker (for example, a piezoelectric speaker) having a flat plate diaphragm performs a sound reproduction, the sound pressure P is proportional to the square of the frequency f. Therefore, in order to obtain a constant sound pressure throughout all bands, the lower the reproduction frequency is, the larger the vibration area a2 and the amplitude x have to be, such that a volumetric capacity from which air is released by the diaphragm is increased.
Focusing on a portabililty, a mobile device such as a mobile telephone or a PDA (personal digital assistant) has such a favorable casing as to allow reductions in thickness, size, and weight of the device. Therefore, the speaker unit is often mounted in a restricted mounting space in a casing of the mobile device. Further, in a case where although the speaker unit is designed for stereo reproduction, the aforementioned space in which the speaker unit is mounted remains unchanged, the width, height, and depth thereof are restricted. When two speakers are mounted on the same plane, each of the speakers has its diameter halved. For example, when each of the speakers has a round shape, the vibration diameter area thereof is reduced to a quarter, thereby exerting a substantial influence on the low frequency reproduction.
Further, in general, since a user is allowed to rotate a display screen mounted on the mobile telephone or the PDA, the user can view a moving picture on the display screen oriented in two directions, i.e., oriented vertically and horizontally. That is, in order to maintain a stereo sound field at sound reproduction regardless of whether the display screen is oriented vertically or oriented horizontally, it is necessary to mount at least three (preferably at least four) speakers. However, since the speaker unit is to be mounted in a restricted space as described above, it is difficult to realize a stereo performance using a diameter of a speaker providing a mono performance as well as to increase the size of the speaker, taking into consideration places into which a plurality of speakers are mounted. That is, although the compact mobile device has to include a unit having a thickness and a diaphragm diameter increased so as to perform the low-frequency reproduction, it is difficult to provide a space for mounting the plurality of speakers capable of performing the stereo reproduction and the like because of a reduced volumetric capacity for mounting the speaker units.
Therefore, an object of the present invention is to provide a piezoelectric speaker, which is mounted in a reduced volumetric capacity, capable of performing both a low-frequency reproduction and a stereo reproduction, and a method for manufacturing the same.